Conventionally, an electrolyte that is formed by impregnating a non-aqueous electrolytic solution into a film having pores in which the film is referred to as a separator has been generally used as an electrolyte of a lithium secondary battery. In recent years, a lithium secondary battery (such as a polymer battery) using a polymer electrolyte made of a polymer other than such a liquid-phase electrolyte has attracted attention.
The polymer battery uses an electrolyte in gel form in which a liquid electrolytic solution has been impregnated in the polymer. Since the electrolytic solution is retained in the polymer, it is difficult for the electrolytic solution to leak out. Therefore, the polymer battery has the advantages that the safety of the battery is enhanced, and also the contour of the battery may be formed freely.
The polymer electrolyte has the low conductivity of the lithium ions when being compared with the electrolyte composed of only the electrolytic solution. Thus, due to the low conductivity of the lithium ions, a method for reducing the thickness of the polymer electrolyte is being used. However, when the polymer electrolyte is reduced to be thin in this way, the mechanical strength of the polymer electrolyte is reduced, and the positive electrode and the negative electrode are short-circuited at the time of manufacture of the battery, to thus cause a problem of easily destroying the polymer electrolyte.
Korean Patent Registration No. 10-0637481 proposed a lithium secondary battery having a positive electrode, a polymer electrolyte, and a negative electrode, wherein the polymer electrolyte is formed by impregnating an organic electrolyte solution in a nonwoven fabric having at least gelling fibers that are easily gelled by the organic electrolytic solution, and non-gelling fibers, wherein the gelling fibers are polyacrylonitrile-vinyl acetate copolymers, in a gelled state including the organic electrolyte solution, wherein a mixture ratio of the gel-like gelling fibers and the non-gelling fibers is 3:97 to 75:25 at a weight ratio, and a content of vinyl acetate is 5 wt % or more to 20 wt % or less.
Since the polymer electrolyte proposed in Korean Patent Registration No. 10-0637481 is formed by impregnating an organic electrolyte solution in a nonwoven fabric having at least gelling fibers and non-gelling fibers, the uniformity of portions of the gelling fibers that are gelled by the organic electrolytic solution may not be guaranteed to thus be unable to guarantee uniform ion conductivity and to thus cause internal short-circuit possibility. In addition, since the polymer electrolyte is in the form of the nonwoven fabric, it is difficult to achieve a uniform thin film although the polymer electrolyte is gelled.
Korean Patent Registration No. 10-1208698 proposed a secondary battery including: two separate different electrodes; a heat-resistant ultrafine fiber-shaped porous separator interposed between the two electrodes and including ultrafine fibers that are obtained by air electrospinning (AES) a mixture solution of a heat-resistant polymer material having a melting point of 180° C. or higher, and a content of 50 wt % to 70 w %, and a swellable polymer material of 30 wt % to 50 w % that is swellable in an electrolytic solution; and the electrolytic solution or an electrolyte.
However, Korean Patent Registration No. 10-1208698 proposed a porous separator including ultrafine fibers that are obtained by spinning a mixture solution of a heat-resistant polymer material and a swellable polymer material, but did not recognize an advantage of fibers having a core-shell structure and a condition of forming the core-shell structure.
In addition, Korean Patent Application Publication No. 10-2012-46092 proposed a heat-resistant separator including a first pore-free polymer film layer, and a porous polymer web layer that is formed of ultrafine nanofibers that are formed on the first pore-free polymer film layer and are formed of a mixture of a heat resistant polymer and inorganic particles or a mixture of a heat-resistant polymer, a swellable polymer and inorganic particles.
Since the heat-resistant separator is a thin film of a two-layer structure having a thickness of 10 to 60 μm, the tensile strength of the heat-resistant separator is low, handling properties are poor during production, and manufacturing costs are high, to thereby cause weak competitiveness. In general, when compared to other fibers, nanofibers have good relative strength, but have weak absolute strength.
In other words, in the case that a separator is made of only nanofibers, a heavy weight of nanofibers of approximately 10 g/m2 is needed in order to make it possible to perform the handling. However, this heavy weight separator is a factor that is directly connected with a production rate, to thereby cause a high production cost.
In addition, since nanofibers have a large amount of static electricity at the time of a manufacturing process, it may cause a very difficult handling problem in itself. The removal of the static electricity is not possible through the composite process such as lamination, but it is possible to improve handling properties.
Furthermore, since a polymer web separator has a porosity of 80% or so and thus movement of ions is performed so well, micro-short may occur to thus cause a phenomenon of OCV (open circuit voltage) degradation.
Since a nonwoven fabric made of a PP/PE (Polyethylene/Polypropylene) or PET (PolyEthylene Terephthalate) fiber has too a high porosity, it is not possible to use the nonwoven fabric alone as a separator. In particular, since the nonwoven fabric has a porosity of 70 to 80%, an OCV characteristic is poor by self-discharge, a large variation in pores occurs, and large-sized pores exist.
When considering the above-described conventional defects, a ceramic layer is added in a nonwoven fabric by mixing inorganic particles with a binder, to thus obtain a separator that reduces the porosity and reinforces the heat resistance. However, such a separator has a problem that the production process is complicated, and the inorganic particles are eliminated.